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Considerations In Applying Geotextiles To Coastal Revetments
The application of geotextile membranes in breakwater and revetment design raises the issue of the appropriate soil/geotextile and geotextile/geotextile friction angles that can be adopted for stability analysis. A considerable amount of data, much derived from the design of landfills, has been published on this subject. Other data are provided by geotextile manufacturers. Much of the data refer to a variety of woven fabrics, but data exist also for non-woven needle punched geotextiles that are used in coastal engineering structures. This paper reviews the local practice and literature and proposes appropriate values for soil/geotextile and geotextile/geotextile friction angles that may be considered for the preliminary design of coastal revetment structures.
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Fibre Reinforced Soils For Geotechnical Infrastructure
This paper presents the results of recent laboratory studies on fibre reinforced soils. Drained and undrained triaxial test results highlight how soil stress-strain behaviour may be altered by mixing with discrete flexible fibres. In triaxial compression a considerable strength increase is induced by the presence of fibres, while in extension the strength increase is very limited. This is attributed to the fibre orientation distribution with respect to the tensile strains developed. Also presented in the paper is a framework for introducing the effects of fibres and their orientation into a constitutive model to describe the anisotropic stress-strain behaviour of fibre reinforced soils. Model simulations of selected test results are shown. Also described are examples of future investigations and trials required to make the soil reinforcement technology ready for use in industry.
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Numerical analysis of the bearing capacity of inclined loaded strip footings supported on sheet pile wall stabilized slopes
In practice, the bearing capacity of a strip footing adjacent to a slope is significantly reduced. This paper aims to control the generated failure mechanism and ameliorate the slope stability, using a sheet pile wall reinforcement technique. A two dimensional finite element limit analysis is used to examine the failure condition, through OptumG2 code. The effect of inclined loading on failure envelopes is investigated, through the average of upper and lower bound solutions. This paper focuses on the estimation of the undrained bearing capacity improvement factor for a given load inclination and slope angle. A new evaluation of the size and shape of failure envelopes is presented. Thus, a comparison between the undrained bearing capacity improvements before and after the sheet pile reinforcement is made, to study the most efficient case. The modification in failure loads are compared with those available in the literature.
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Uncertainty, economic risk analysis and risk acceptance criteria for mine subsidence
There are often differences in ‘expert opinions’ of what constitutes appropriate design parameters for determining the level of stability of the existing pillars of old coal mine workings. For example, there is seldom 100% certainty about the value of any design parameter. Instead there are varying degrees of confidence (or belief) for each possible design value. This uncertainty in design parameters can be represented by probability distributions. The inclusion of such probabilistic information into a probabilistic risk analysis will enable the probability of failure to be estimated. To illustrate the utility of risk analysis for decisions taken with uncertainty, a probabilistic risk analysis has been conducted to assess the uncertainty of design parameters on the level of stability of existing pillars within a disused coal seam beneath a proposed surface development in the Newcastle area. The case study considers dimensional and level of inundation uncertainties. This case study provides a preliminary framework for a risk-based approach to decision- making for a geotechnical system subject to high uncertainties. The outcomes of the risk analysis are probability of failure and annual economic risks (expected losses per year). The paper will describe the steps taken in the risk assessment, risk acceptance criteria and how results from a risk analysis may be interpreted by a decision-making development consent authority.
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A preliminary study of strength behaviour of lime-slag treated pyrite bearing soft Coode Island Silt
The prime locations of the central business district of Melbourne are underlain by extensive deposits of a very soft and highly compressible soil, locally known as Coode Island Silt (CIS). This soft deposit poses serious challenges for the design and construction of economic foundations. The conventional practice for almost any construction over this soft soil is to use pile foundations extended to Melbourne mudstone layer situated at about 30 m depth. Results from recent researches on employing soil mixing technique to improve the engineering properties of this soft soil by using different additives have been found to be promising. Pyrite bearing CIS was treated with lime-activated slag of various proportions. Presence of pyrite is responsible for creation of acidic environment through its oxidation and thereby hindering the progress of cementitious reactions. Through experimental study it has been found that providing higher amount of lime can be an effective way of overcoming the adverse effect of pyrite. Initial Consumption of Lime (ICL) test can be a useful tool to determine the minimum amount of lime required for the stabilization to be effective. The results of experimental investigations, consisting of unconfined compressive strength tests and XRD analyses, on soft CIS treated with lime-activated slag are presented in this paper. In addition, the importance of carrying out ICL test for the selection of additive dosage is highlighted by presenting the results of strength tests of CIS treated with lime contents both above and below ICL.
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Concrete injection column ground treatment design within a brownfield site with compressible soils
Concrete Injection Columns (CICs) are commonly used in geotechnical ground treatment to meet serviceability and stability design criteria. Together with a load transfer platform, the design objectives are achieved by bridging future fills across existing fills and soft materials to transfer loads from a higher level to a stronger underlying material layer. Efficient CIC design within brownfield sites can be challenging when there is high variability or uncertainty in the depths and extents of existing fills and soft materials. This case study presents the CIC ground treatment design development and validation required to support the future rail earthworks platform for the Sydney Metro West – Western Tunnelling Package Clyde Stabling & Maintenance Facility (SMF). The selected performance based geotechnical ground treatment design comprised 0.45m diameter CICs at typical 2m centre to centre (c/c) spacings to overcome the risk of excessive site wide settlements and meet the project settlement criteria. Validation included a CIC installation trial to calibrate CIC rig torque readings intersecting soft materials and a static load test to demonstrate CIC settlement performance in line with predicted settlement behaviour, reducing uncertainty in the installation methodology and production CIC performance prior to construction.
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Application of advanced in situ testing equipment and methodologies to characterise a low strength soil deposit in Sydney Harbour
In situ testing equipment and methodologies have evolved rapidly over the last 5 years. This paper looks at three devices that demonstrate this evolution: (1) a Special Purpose 3MPa CPTu Cone; (2) the automated Medusa flat plate dilatometer; and (3) the automated down-the-hole Vane Shear device. CPTu cones are now commercially available with special internal design and with capacities as low as 3MPa. These “special” cones, when carefully calibrated, are capable of reliably measuring CPTu parameters in soils right to the bottom end of the very soft range. The cones come with a temperature sensor to enable the management of potential inaccuracies associated with transient temperature effects during the penetration. The Medusa DMT, was developed by Marchetti to provide technicians and engineers with complete control and repeatability of the DMT diaphragm inflation and measurement process, eliminating many of the operator-dependant variables often encountered with the traditional gas-operated DMT. The Vane Shear Test is perhaps the most relied-upon geotechnical strength test; however traditionally it has had in-built potential errors, particularly in very soft soils, mainly due to ambiguity in friction corrections; it has been traditionally restricted to use in pure clay or clay-like soils. Equipment design has evolved (in some equipment) to eliminate the friction-correction problem. High quality calibration is required. These advanced in situ testing tools and improved methodologies were utilised in combination to characterise a low strength soil deposit in Sydney Harbour for a major infrastructure project. The results from the use of the advanced equipment and methods are discussed and reviewed in this paper.
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Soil-Structure Interaction Of Battered Minipile Groups In Sandy Soil
Battered minipile groups mimicking tree root networks have been gaining popularity as a footing solution for light structural applications in residential, commercial and infrastructure sectors, recently. Battered minipile group configurations are recently in the limelight due to advantages such as ease of installation and environmentally friendly nature. The lateral load resistance of battered minipile groups is investigated in this paper through a combination of physical and numerical modelling. Two-unconventional battered minipile groups with configurations representing the root network of trees with the capacity of engaging a larger volume of soil compared to conventional battered minipile group configurations are studied. A conventional battered minipile group is also included in the study to draw a direct comparison with the new minipile group configurations introduced in this paper. The conventional battered minipile group has two positively and two negatively 25° battered minipiles. The second type of group has one 25° perpendicularly battered minipile in the leading and trailing row each. Another unique orientation of the battered minipile group is also introduced in this study which has four diagonally outward 25° battered minipiles. The third type of minipile group with four diagonally outward battered minipiles offered the highest lateral resistance among the three groups. This better performance capability was attributed to the engagement of a larger volume of soil in resisting lateral load applied at the minipile head. Through this study, the industrial application of the unconventional minipile group configuration with better performance capability in terms of lateral load resistance can be advocated more confidently.